WO2002075378A2

WO2002075378A2 - Tuneable optical filter arrangement for wavelength division multiplex (wdm) network

Info

Publication number: WO2002075378A2
Application number: PCT/GB2002/001196
Authority: WO
Inventors: Robert Charles Goodfellow
Original assignee: Marconi Uk Intellectual Property Ltd
Priority date: 2001-03-17
Filing date: 2002-03-14
Publication date: 2002-09-26
Also published as: WO2002075378A3

Abstract

A tuneable optical filter arrangement (54) for selecting a wavelength channel from radiation including a plurality of wavelength channels in a WDM network is disclosed. The filter arrangement comprises a moveable planar support member (58) which carries a plurality of filter elements (60a 60x). Each filter element (60a 60x) comprises a multi-layered dielectric stack (multiple Fabry-Pérot etalon) having a fixed passband corresponding a WDM wavelength channel. In operation a wavelength channel is selected by moving (translating or rotating) a respective filter element into the path of said radiation.

Description

TUNEABLE OPTICAL FILTER ARRANGEMENT FOR WAVELENGTH

DIVISION MULTIPLEX (WDM) NETWORK

This invention relates to a tuneable optical filter arrangement for selecting a wavelength channel from radiation including several in a wavelength division multiplex (WDM) communication network.

Selection of one wavelength channel from several being carried on an optical fibre is an important function within WDM communications networks, such as for example dropping a selected wavelength channel at a particular node within the network. An example of a known arrangement for dropping two wavelength channels denoted λi, λ₂ from an optical fibre 2 carrying "n" WDM wavelength channels λ_\- λ_n is shown in Figure 1. The arrangement comprises a respective fixed wavelength dichroic mirror 4, 6 for selecting (dropping) a respective wavelength channel λ_ls λ₂. Each dichroic mirror transmits radiation (light) of a selected wavelength range (wavelength channel) and reflects radiation of all other wavelengths. Light from the optical fibre 2 carrying the various WDM channels is collimated and directed by a lens 8 onto the first mirror 4 whose optical characteristic is such that it transmits light having a wavelength corresponding to the first wavelength channel λ^d reflects light having wavelengths corresponding to the other WDM wavelength channels λ₂, λ₃... λ„. The light which is transmitted by the mirror 4 is coupled by a lens 10 into an optical fibre 12. Light which is reflected by the first mirror 4 is arranged to then be incident upon the second dichroic mirror 6 which in a like manner transmits light corresponding to the second wavelength channel λ₂ which is then coupled into a respective optical fibre 16 via a focussing lens 14. The light reflected from the second mirror 6 is then coupled back into the optical fibre 2 by a further lens 18.

Typically each dichroic mirror comprises a multiple-layered (often termed thin film) dielectric structure in which the thin film layers form a series of coupled optical resonators (Fabry-Perot etalons). The transmission passband of each mirror is determined by the thicknesses of the various layers, their refractive index and the reflectivity of the dielectric layers defining the etalon cavities. By careful design it is possible to fabricate mirrors having a 50GHz passband bandwidth, an insertion (transmission) loss of 0.6 to IdB and a rejection of >35dB for wavelengths outside the transmission passband.

As an alternative to dichroic mirrors it is also known to use a fibre Bragg grating 20 which reflects light at the selected channel and transmits light for all other wavelengths, Figure 2. To isolate the selected channel an optical circulator or Mach Zehnder interferometer 22 is provided upstream of the grating 20.

Although such filtering arrangements are widely used in WDM communications networks to drop a selected WDM channel at a given node they have an inherent limitation of being inflexible in that only a single wavelength channel can be dropped at each node and this is fixed by the physical characteristics of the respective mirror or grating. To enable future expansion of WDM networks and to enable switching of selected channels to alternative nodes in the event of component failure, it is highly desirable to be able to remotely reconfigure the network such that nodes can be switched to detect any selected wavelength channel. Such flexibility requires a tuneable wavelength filter or mirror whose optical characteristic can be tuned to select a wavelength channel. Although it is possible to wavelength tune a dichroic mirror, by pivoting it such as to vary the angle of incidence of light thereupon, and thereby change the effective cavity lengths, this is in general used to fine tune the mirror rather than tuning it to a different wavelength channel. To enable tuning between wavelength channels requires a large number of Fabry-Perot etalons and for a 10GHz resolution as is required in a 64 channel lOGbitsS^"1 WDM System this would require several tens of angular degrees of tuning which can result in the passband varying in an undesirable way. Similarly a fibre Bragg grating can be tuned to a limited extent by applying physical strain to the grating or heating it such as to alter the grating period.

The present invention has arisen in an endeavour to provide a tuneable optical filter for use in a WDM communications network which in part, at least, overcomes the limitations of the known filters.

According to a first aspect of the present invention a tuneable optical filter arrangement for selecting a wavelength channel from radiation including a plurality of wavelength channels in a WDM network comprises: a plurality of moveable filter elements, wherein each filter element comprises a multiple-layered dielectric stack having a fixed passband corresponding to a WDM wavelength channel and wherein in operation a wavelength channel is selected by moving a respective filter element into the path of said radiation. The wavelength channel which is selected may be at least substantially transmitted by its respective filter element in the filter arrangement. The wavelength channels which are not selected may be at least substantially reflected by the filter element.

Advantageously the filter elements are mounted on a moveable support member, which may be a movable planar support member. The support member may be moved to place a filter element in the path of the radiation. Preferably the support member is translatable in at least one direction. In one arrangement the support member is translatable in a single plane in two mutually orthogonal directions and the plurality of filter elements are arranged as an array on a planar surface of said support member. Alternatively, or in combination with being translatable, the support member may be rotatable.

In one arrangement the support member has a plurality of apertures extending through its entire thickness and each filter element is mounted such as to overlay a respective aperture. Alternatively the support member is solid and is made of a material which is substantially transparent to radiation of all wavelength channels. In such an arrangement the filter elements are bonded or fixed to a planar surface of said support member using for example an optical quality adhesive such as an epoxy or Canada balsam or other suitable bonding or clamping arrangement. The filter elements may be positioned on the support member such that substantially no space is left between them. Alternatively, the filter elements may be positioned on the support member such that spacing exists between them. In such a case, the spacings may be substantially reflective to all wavelength channels. The spacings may be provided by the support member, which may be made of a material which is substantially reflective to all wavelength channels, or may be provided with a reflective layer such as a layer of gold. Either arrangement is particularly advantageous during retuning of the filter arrangement, i.e. when the selected filter element is being changed. As the filter elements are being moved, if there is no spacing between them or reflective spacings between them, reflection of the wavelength channels not selected by the elements will be maintained. The filter elements or some of the filter elements may have dimensions of substantially 1.4mm by substantially 1.4mm or substantially 1mm by substantially 1mm.

To limit the effects of temperature on the optical performance of the filter arrangement the support member and filter elements preferably comprise materials having coefficients of thermal expansion which are substantially the same.

Preferably the filter elements comprise multiple-layers selected from the oxide glasses of silicon (silica), titanium (titania), tantalum, aluminium (alumina), magnesium, vanadium, lanthanum, manganese, lead, tungsten, zirconium (zirconia) or mixtures of such oxide glasses. Advantageously each filter element has a passband attenuation of less than 1 dB, preferably less than 0.5dB.

According to a second aspect of the invention an arrangement for selecting a wavelength channel from radiation which includes a plurality of wavelength channels, said radiation being carried by an optical fibre, comprises: a radiation shaping arrangement, a tuneable optical filter arrangement as described above in which the plurality of filter elements is selectively moveable into the path of the radiation, and a receiver, wherein the radiation shaping arrangement receives the radiation carried by the fibre and shapes this into a beam comprising at least a portion having a wavefront which is substantially planar, the beam is directed onto a selected filter element of the filter arrangement such that the substantially planar wavefront portion is incident on the selected filter element, and the filter arrangement provides an optical output signal which is directed to the receiver.

The radiation shaping arrangement may comprise the output end of the fibre which has been modified such that it produces a beam of radiation having a substantially planar wavefront. The modification may comprise treating the end of the fibre such that the core thereof dissolves into the cladding, effectively forming a graded lens from the end of the fibre which emits a beam of radiation having a substantially planar wavefront.

The radiation shaping arrangement may comprise one or more lens arrangements.. The lens arrangement or arrangements may be provided separately from the fibre, or may be attached to the fibre, or a combination of both. The lens arrangement or arrangements may act to collimate the radiation carried by the fibre into a beam of radiation having a substantially planar wavefront and to direct the beam onto the selected filter element. The radiation emitted by the optical fibre may have a substantially gaussian-shaped intensity profile. The lens arrangement or arrangements may act to shape the radiation into a beam of radiation having a beam-waist and to direct the beam onto the selected filter element, such that the beam-waist is incident on the filter element. (The beam- waist will have a substantially planar wavefront). The dimensions of the filter element may be chosen such that the whole or substantially the whole of the beam-waist is incident on the filter element. As the beam-waist has a reduced cross sectional area relative to the remainder of the beam, the filter element dimensions will be less than those required if the element were to be positioned such that a different portion of the beam were incident thereon, and the angular spread of radiation will be effectively zero, so the filter element will provide a narrow passband.

The receiver may comprise an optical fibre. This fibre may receive the optical output signal of the filter arrangement, and may direct this signal to an optical to electrical converter. Alternatively, the receiver may comprise an optical to electrical converter. In either case, the optical to electrical converter may comprise a photodetector. The photodetector may receive the optical output signal of the tuneable optical filter arrangement, and convert this to an electrical output signal. The optical to electrical converter may comprise a preamplifying electronic circuit. This circuit may receive the electrical output signal of the photodetector. The preamplifying electronic circuit is preferably capable of receiving and processing signals comprising 2.5 Gbit per sec data, or 10 Gbit per sec data or 40 Gbit per sec data or any combination of such signals.

The arrangement for selecting a wavelength channel may comprise apparatus for positioning and holding the filter elements of the tuneable optical filter arrangement. The apparatus may position and hold a selected filter element such that the angle between the substantially planar wavefront portion of the beam and the filter element is substantially 90°. Radiation comprising the wavelength channels which are not selected by the filter element may then be reflected from the filter element at an angle of substantially 180°. Such radiation may thus be directed back into the optical fibre.

Alternatively, the apparatus may position and hold the selected filter element such that the angle between the substantially planar wavefront portion of the beam and the filter

element is other than 90°. Radiation comprising the wavelength channels which are not

selected by the filter element may then be reflected from the filter element at an angle

other than 180°. Such radiation may, for example, be directed into the end of another

optical fibre.

The arrangement for selecting a wavelength channel may comprise a lens arrangement which directs the optical output signal of the filter arrangement to the receiver. This lens arrangement may act to shape the optical output signal of the filter arrangement before directing it to the receiver.

The arrangement for selecting a wavelength channel may comprise a beam bending system. This system may act to bend the radiation carried by the fibre and emitted therefrom, or the optical output signal of the tuneable optical filter arrangement, or the radiation comprising the wavelength channels which are not selected by the filter element of the filter arrangement, or some or all of these. The system may bend the radiations or the signal in such a way that the overall size of the arrangement for selecting a wavelength channel is minimised. The arrangement may then be incorporated in the type of flat, compact package often required for, for example, wavelength division multiplex communication networks. The beam bending system may comprise one or more mirror systems. In one embodiment, the beam bending system comprises a mirror placed between the fibre carrying the radiation and the filter arrangement. The mirror is preferably positioned at an angle of substantially 45° to the

radiation, and preferably reflects the radiation through an angle of substantially 90° onto

the filter arrangement. The mirror may be a highly reflective metallic mirror, for example a gold mirror. Such a mirror minimises changes to the polarisation of the radiation incident thereon, causing all states of polarisation to be equally reflective.

The arrangement for selecting a wavelength channel may comprise one or more shutters. For example, a shutter may be placed between the radiation shaping arrangement and the tuneable optical filter arrangement. The shutter, when operated, may be used to prevent radiation from the fibre being directed onto the filter arrangement. The shutter may be of a size substantially equal to or greater than the size of the filter elements. Particularly, the shutter may be used to prevent radiation from the fibre being directed onto the filter arrangement during retiming thereof, i.e. when the selected filter element is being changed. For example, the filter elements may be positioned in a linear array and it may be desired to change the selected filter element from filter element n to filter element n+2. To do so it will be necessary to translate the filter array, and filter element n+1 will pass through the optical path of the filter arrangement. Radiation transmitted by the filter element n+1 may be directed onto the receiver, which may be undesirable. The shutter may be used to prevent radiation from the fibre being directed onto filter element n+1.

The arrangement for selecting a wavelength channel may comprise at least one attenuation system for attenuating the ' radiation carried by the fibre and emitted therefrom, or the optical output signal of the tuneable optical filter arrangement, or the radiation comprising the wavelength channels which are not selected by the filter element of the filter arrangement, or some or all of these. The attenuation system may comprise a shutter which is partially placed in the path of the radiation or signal and prevents onward transmission of part of the radiation or signal. The attenuation system may comprise means for aligning the selected filter element of the filter arrangement such that it is only partially illuminated by the radiation carried by the fibre, and the output of the filter arrangement is thereby attenuated.

According to a third aspect of the invention a wavelength division multiplex communication network incorporates at least one tuneable optical filter arrangement as described above.

According to a fourth aspect of the invention a wavelength division multiplex communication network incorporates at least one arrangement for selecting a wavelength channel as described above.

The wavelength division multiplex communication network may comprise a coupling arrangement for coupling the wavelength channels which are not selected by the or each tuneable optical filter arrangement into said communication network. The or each or some of the coupling arrangements may comprise an optical circulator. The or each or some of the coupling arrangements may comprise setting up an arrangement for selecting a wavelength channel such that the filter elements of the tuneable optical filter arrangement are tilted with respect to the radiation from which a wavelength channel is to be selected. The angle of incidence of the radiation on the filter elements is preferably in the range 0.25° to 6°. The wavelength division multiplex communications

network may comprise a series of tuneable optical filter arrangements and/or arrangements for selecting a wavelength channel. Such arrangements may be coupled together using one or more of the coupling devices. Radiation may be fed to a first arrangement, where a wavelength channel is selected. Radiation comprising the wavelength channels which are not selected by the first arrangement may be coupled into the communication network and fed thereby to a second arrangement. The second arrangement may select a wavelength channel, different from that selected by the first arrangement, and radiation comprising the wavelength channels which are not selected may be coupled into the communication network and fed thereby to a third arrangement, and so on. The communications network may therefore comprise a series of filter arrangements which are 'daisy-chained' together.

According to a further aspect of the invention in a wavelength division multiplex optical communication system a method of selecting a wavelength channel from radiation including a plurality of wavelength channels comprises: providing a wavelength tuneable filter whose optical passband can be selectively tuned to a selected wavelength channel and directing said radiation through said filter; characterised in that the filter comprises a respective filter element for each wavelength channel wherein each filter element comprises a multiple-layered dielectric stack having a fixed wavelength passband corresponding to a wavelength channel and wherein a wavelength channel is selected by moving the respective filter element into the path of the radiation.

In order that the invention can be better understood two filter arrangements in accordance with the invention will now be described by way of example only with reference to the accompanying drawings in which:

Figure 1 is a schematic representation of a known filter arrangement for dropping two wavelength channels from a WDM network;

Figure 2 is a schematic representation of a further known filter arrangement for dropping wavelength channels from a WDM network;

Figure 3 is a schematic representation of a WDM network node incorporating tuneable optical filter arrangements in accordance with the present invention;

Figure 4 is a schematic representation of a first embodiment of an arrangement for selecting a wavelength channel in accordance with the invention;

Figure 5 is a schematic representation of a second embodiment of an arrangement for selecting a wavelength channel in accordance with the invention;

Figure 6 is a schematic representation of a third embodiment of an arrangement for selecting a wavelength channel in accordance with the invention;

Figure 7 is a schematic representation of a fourth embodiment of an arrangement for selecting a wavelength channel in accordance with the invention; Figure 8 is a schematic representation of a fifth embodiment of an arrangement for selecting a wavelength channel in accordance with the invention; and

Figure 9 is a schematic representation of a WDM communication network in accordance with the invention.

Referring to Figure 3 there is shown a node 40 of a WDM communications network which incorporates an arrangement for selecting a wavelength channel of the present invention. Typically the communications network comprises an eight node WDM optical ring network in which one or more wavelength channels λ_x can be added or dropped at each node 40. At each node an optical amplifier 42 (typically an erbium doped fibre amplifier) is provided to amplify all wavelength channels λ_\- λ_n being carried on an optical fibre 44 of the WDM ring. A device 46 is provided in the path of the fibre 44 and is operable to block all wavelength channels which are to be dropped at the node 40 whilst allowing all remaining channels to be pass. Typically the device 46 comprises in order a wavelength de-multiplexing element (grating) to split the WDM signal into its constituent channels such that each falls on a respective optical shutter (typically an array of liquid crystal shutters) and an optical multiplexer to recombine all wavelength channels passed by the optical shutter. To ensure that the optical signal power level of all of the wavelength channels that are passed is substantially the same as those being added at the node, the device 46 provides a variable level of attenuation to wavelength channels passing through it.

On the input side of the device 46 there is provided an optical coupler 48 for splitting off a fraction of the radiation passing along the fibre 44. This radiation is further amplified by a second optical amplifier 50 before being divided into a plurality of fibres

52a-52x. Radiation from each fibre 52a-52x is applied to a respective tuneable optical filter arrangement 54a-54x and a respective receiver circuit 56a-56x.

On the output side of the device 46 a second 3dB optical coupler 58 is used to add selected wavelength channels to the network fibre 44. A further optical amplifier 60 is used to amplify radiation at all wavelength channels before onward transmission to the next WDM node.

Each tuneable optical filter arrangement 54 is capable of being tuned such as to drop a selected wavelength channel λ_* by tuning its transmission passband to correspond with the selected wavelength channel. It will be appreciated therefore that any receiver 56a - 56x can be readily reconfigured to detect any selected wavelength channel.

Referring to Figure 4 there is shown a schematic representation of a first embodiment of an arrangement for selecting a wavelength channel in accordance with- the present invention, for selecting a wavelength channel from an optical fibre 52 carrying WDM radiation of many wavelength channels and converting it into a corresponding electrical signal. As described above the optical fibre 52 will typically be located at a node within a WDM network though it will be readily appreciated that the arrangement can be used elsewhere within the network to select a wavelength channel.

The arrangement comprises a tuneable optical filter arrangement 54 which comprises a moveable planar support member 58 which carries a plurality of filter elements 60a - 60x on one of its planar faces. The support member 58 and filter elements 60a - 60x can be translated in a direction denoted "x" in the Figure by means of a drive arrangement 62. The drive arrangement 62 can comprise for example a piezoelectric actuator, electrostatic MEMS actuator, lead screw and motor arrangement, inch worm or any other arrangement which is capable of translating the support member 58 to a selected position. A lens 64 is provided to collimate radiation 66 from the optical fibre 52 into a beam 68 having a substantially planar wavefront which is directed onto the support/filter element assembly in a direction substantially perpendicular to the direction "x". Radiation which passes through the support and filter elements is focussed by a lens 70 onto a receiver comprising a photodetector 56 which converts the radiation into a corresponding electrical signal. In operation the wavelength channel being detected by the photodetector 56 is selected by translating the support 58 such that the beam 68 is incident on the appropriate filter element 60a-60x.

In the embodiment illustrated in Figure 4 the support member 58 has a series of apertures 72a - 72x which are spaced in a linear relationship to each other in the direction "x". Each aperture 72a - 72x passes through the entire thickness of the support 58 thereby defining a window there through. A respective planar filter element 60a - 60x is mounted to the support member 58 around the periphery of one of its planar faces such that it overlies its respective aperture 72a - 72x. The spacings between the filter elements are provided by the support member 58, which is made of a material which is substantially reflective to all wavelength channels. Each filter element 60a - 60x preferably comprises a multiple-layered dielectric stack having a fixed wavelength transmission passband in which the centre wavelength of the passband is selected to correspond to one of the WDM wavelength channels. The passband bandwidth is selected such that taking account of modulation of the channel it is sufficiently wide enough to allow the wavelength channel to pass substantially unattenuated, but narrow enough to block neighbouring channels. For clarity only five filter elements 60a - 60c and 60x are shown in Figure 4 though it will be appreciated that a filter element will typically be provided for each WDM wavelength channel e.g. thirty two, sixty four for current and proposed WDM networks. Radiation blocked by

the filter element is reflected therefrom at an angle of substantially 180° back along the incident path, through the lens 64, and back into the optical fibre 52. This radiation comprises the wavelength channels not selected by the filter element.

As is known, the filter elements 60 are fabricated by depositing various material layers onto a planar substrate which is made of a material which is substantially transparent to light having wavelengths corresponding to any one of the WDM wavelengths. For "C" band operation, that is 1535nm - 1560nm, the substrate is preferably sapphire or silica glass, though other materials such as barium oxide, strontium oxide can be used. The various layers of dielectric material comprising the filter element 60 are of uniform thickness with alternate layers being of high and low reflectivity materials such as for example layers of zirconia (zirconium oxide) and titania (titanium oxide). It will be appreciated that the various layers therefore constitute a series of coupled optical resonators, Fabry-Perot etalons. In a known manner the thicknesses of the various layers are selected such as to give as flat a passband as possible with the centre wavelength corresponding to one of the WDM channels. The filter elements 60 are accurately placed on the support member 58 overlying their respective aperture 72 using a "pick-and-place" machine and are bonded or fixed to the support using an adhesive, solder or other suitable bonding or fixing (e.g. clamping) arrangement. It is important to ensure that each element 60 is bonded to the support such that the material layers within the element 70 are within a pre-determined angular relation to the surface of the support member 58, in this embodiment this is as parallel as practicable to ensure that the beam 68 of radiation, more particularly the wavefront of the radiation, strikes the element 60 at near-normal incidence. In practice it is found that a quarter degree variation in angular orientation of the filter element to the beam results in a wavelength shift of approximately 25 GHz in the passband of the element (more particularly a 0.6nm variation per angular degree of change). Thus for a WDM system having a wavelength channel spacing of 50GHz each of the filter elements should be oriented to an accuracy of better than 12 minutes (5GHz) for filter elements having a passband of 25GHz.

In this embodiment, the dimensions of the filter elements 60 are selected such that the entire extent of the beam 68 can be filtered. It will be appreciated therefore that, unlike the known tuneable filter, this filter will have a uniform passband across the entire extent of the beam 68 provided that the beam has a planar or near planar wavefront. Additionally, each filter element will have a low insertion loss since each is fabricated such as to optimise its characteristic for only a single wavelength channel. To minimise the overall dimension of the support member 58, which it will be appreciated has to be capable of being translated by an amount corresponding to substantially its entire length to enable any element to be selected, the filter elements are typically square and of dimensions 1 to 1.5mm and may be spaced in as close a relationship to one another as is practical. For ease of fabrication the filter elements are preferably fabricated in the form of larger wafers which are then sliced or cleaved into elements of the required dimensions.

Referring to Figure 5 there is shown a schematic representation of a second embodiment of an arrangement for selecting a wavelength channel in accordance with the invention. For clarity like reference numerals are used to denote like parts. In this embodiment, the filter elements 60a - 60x are arranged near the circumference of a circular support member 58.

Referring to Figure 6 there is shown a schematic representation of a third embodiment of an arrangement for selecting a wavelength channel in accordance with the invention. For clarity like reference numerals are used to denote like parts. In this embodiment, the radiation emitted by the optical fibre 52 has a substantially gaussian-shaped intensity profile. The lens 64 acts to shape the radiation into a beam of radiation having a beam- waist 80. This beam of radiation is directed onto a selected filter element 60n of a tuneable optical filter arrangement 54, such that the beam-waist is incident on the filter element. The beam-waist has a substantially planar wavefront, and strikes the filter element 60n at near-normal incidence. Radiation which passes through the filter element is focussed by lens 70 into a second optical fibre 81. Radiation from this optical fibre is directed onto a photodetector 56. The arrangement also comprises a shutter 82, placed between the lens 64 and the filter element 60n. It will be appreciated that the movable planar support member 58 can be translated to place a selected filter element in the path of the beam of radiation from the optical fibre 52. The dimensions of the filter elements are such that the whole of the beam of radiation 68 is filtered. As can be seen however, as the beam-waist 80 has a reduced cross-sectional area in comparison to the remainder of the beam, the filter element dimensions in this embodiment can be less than those of the other embodiments shown.

Referring to Figure 7 there is shown a schematic representation of a fourth embodiment of an arrangement for selecting a wavelength channel in accordance with the invention. For clarity like reference numerals are used to denote like parts. In this embodiment, radiation is emitted from the optical fibre 52 and is incident on a filter element 60n of a tuneable optical filter arrangement 54. The filter element is positioned at an angle of

approximately 45° to the beam 68. Radiation which passes through the filter element is

focussed by lens 70 into an optical fibre 81. Radiation 85 reflected from the filter element is focussed by lens 86 into a optical fibre 87. The radiation reflected by the filter element will comprise the wavelength channels which are not selected by the filter element 60n.

Referring to Figure 8 there is shown a schematic representation of a fifth embodiment of an arrangement for selecting a wavelength channel in accordance with the invention. For clarity like reference numerals are used to denote like parts. In this embodiment, radiation emitted from the optical fibre 52 is incident on a beam bending system. This comprises a gold mirror 90 placed between the fibre 52 and a tuneable optical filter

arrangement 54, at an angle of approximately 45° to the beam of radiation 68. The

mirror 90 bends the beam through approximately 90°, and directs it onto a filter element 60n. The overall size of the arrangement for selecting a wavelength channel is thus minimised, and the arrangement may be incorporated into a flat, compact pack for use in a WDM communication network.

Referring to Figure 9 there is shown a schematic representation of a WDM communication network in accordance with the invention. For clarity like reference numerals are used to denote like parts. In this, radiation from the network is fed into a first optical circulator 95, and from there to a first arrangement for selecting a wavelength channel 96. The arrangement 96 selects a wavelength channel which passes through a filter element 60n and is focussed into an optical fibre as shown. Radiation comprising wavelength channels which are not selected by the filter element is reflected therefrom, and is directed back through the optical fibre 52 to the optical circulator 95. This directs the radiation to a second optical circulator 97, via an optical fibre 98. The optical circulator 97 directs the radiation to a second arrangement for selecting a wavelength channel 99, via an optical fibre 100. The arrangement 99 selects a wavelength channel, different from that selected by the arrangement 96. Radiation comprising the wavelength channels which are not selected is reflected from the filter element 60n+l of the arrangement 99, and is directed back through the optical fibre 100 to the optical circulator 97, and from there to a third arrangement (not shown) and so on. The WDM communication network therefore comprises a series of arrangements for selecting a wavelength channel which are 'daisy-chained' together.

It will be appreciated that the arrangements for selecting a wavelength channel and the tuneable optical filter arrangements of the present invention are not restricted to the embodiments described and that variations can be made which are within the scope of the invention. For example, whilst to minimise the insertion loss of the filter arrangement it is preferred to have apertures through the support member over which the filter elements lie, it is also envisaged to used a solid support made of a material which is transparent to all the WDM wavelength channels (such as silica glass or sapphire) and to bond the elements onto the surface of the support. In such an arrangement the filter elements can be bonded using an optical quality adhesive such as an epoxy resin or Canada balsam. During bonding it is preferred to temperature cycle the assembly to minimise the formation of air bubbles in the bond and thereby minimise the insertion loss for each filter element. A benefit of using a solid support member is that each filter element is fully supported over its entire surface which reduces the likelihood of distortion of the elements due to, for example, differential thermal expansion of the support member and elements. As will be appreciated distortion of the filter elements can result in a shift in the passband characteristic of the element, or a de- tuning of the element such that its optical characteristic varies with position. To minimise the effects of temperature therefore it is preferable that the support and filter elements are made of materials of similar or identical coefficient of thermal expansion or that the support is sufficiently compliant that strain effects are minimised. Although the filter elements are shown orientated such that the layers thereof are substantially parallel to the planar surface of the support member, other orientations are possible.

Whilst the filter elements have been described as comprising multiple-layers of silica and zirconia other thin film dielectric stacks can be used such as those including the oxide glasses of titanium (titania), tantalum, aluminium (alumina), magnesium, vanadium, lanthanum, manganese, lead, tungsten, zirconium (zirconia) or mixtures of such glasses.

Furthermore it will be readily appreciated by those skilled in the art that other mechanical arrangements can be used to move a selected filter element within the beam of radiation such as to select a required wavelength channel. For example it is also envisaged to have each of the filter elements independently moveable such that a given filter is moved within the path of the beam of radiation. In addition for systems having a large number of wavelength channels it is preferred to arrange the elements as an array and to translate the support in two orthogonal directions. This has the benefit of reducing the overall distance the support member needs to be translated in a given direction.

Throughout the foregoing description the filter elements have been described as having a transmission passband which is engineered to correspond with one of the wavelength channels. It will be appreciated that, in other embodiments, the filter elements can have an optical characteristic which reflects a given band of wavelengths corresponding to a selected WDM channel.

A particular benefit of the tuneable optical filter arrangement of the present invention is that it retains all of the benefits of fixed wavelengths filters whilst offering the flexibility of being remotely reconfigured without requiring a site visit. A further advantage of the filter arrangement of the present invention is the reduction in the number of spares that need to be carried. For example, at present each receiver card at a node within a thirty wavelength channel WDM system requires a corresponding number of fixed wavelength filters and so spares. In contrast, in the present invention, only a single filter arrangement is required which is common to each receiver card.

Claims

1. A tuneable optical filter arrangement (54) for selecting a wavelength channel from radiation (68) including a plurality of wavelength channels in a WDM network characterised by comprising: a plurality of moveable filter elements (60a-60x), wherein each filter element comprises a multiple-layered dielectric stack having a fixed passband corresponding to a WDM wavelength channel and wherein in operation a wavelength channel is selected by moving a respective filter element into the path of said radiation (68).

2. A tuneable optical filter arrangement (54) according to Claim 1 characterised by further comprising a moveable support member (58) on which the filter elements (60a-60x) are mounted.

3. A tuneable optical filter arrangement (54) according to Claim 2 characterised in that the support member (58) is translatable in at least one direction.

4. A tuneable optical filter arrangement (54) according to Claim 3 characterised in that the support member (58) is translatable in a single plane in two mutually orthogonal directions and in which the plurality of filter elements (60a-60x) are arranged as an array on a planar surface of said support.

5. A tuneable optical filter arrangement (54) according to any of Claims 2 to 4 characterised in that the support member (58) is rotatable.

6. A tuneable optical filter arrangement (54) according to any of Claims 2 to 5 characterised in that the moveable support member (58) has a plurality of apertures (72a-72x) extending through its entire thickness and in which each filter element is mounted such as to overlay a respective aperture.

7. A tuneable optical filter arrangement according to any of Claims 2 to 5 characterised in that the support member is solid and is made of a material which is substantially transparent to radiation of all wavelength channels.

8. A tuneable optical filter arrangement according to Claim 7 characterised in that the filter elements are bonded to a planar surface of said support member.

9. A tuneable optical filter arrangement according to any of Claims 2 to 8 characterised in that the filter elements are positioned on the support member such that substantially no space is left between them.

10. A tuneable optical filter arrangement (54) according to any of Claims 2 to 8 characterised in that the filter elements (60a-60x) are positioned on the support member (58) such that spacing exists between them.

11. A tuneable optical filter arrangement (54) according to Claim 10 characterised in that the spacings are substantially reflective to all wavelength channels.

12. A tuneable optical filter arrangement according to Claim 11 characterised in that the spacings are provided by the support member, which is provided with a reflective layer.

13. A tuneable optical filter arrangement (54) according to any of Claims 2 to 12 characterised in that the support member (58) and filter elements (60a-60x) comprise materials having coefficients of thermal expansion which are substantially the same.

14. A tuneable filter arrangement (54) according to any preceding claim characterised in that the filter elements (60a-60x) comprise multiple layers selected from the oxide glasses of silicon (silica), titanium (titania), tantalum, aluminium (alumina), magnesium, vanadium, lanthanum, manganese, lead, tungsten, zirconium (zirconia) or mixtures of such oxide glasses.

15. A tuneable filter arrangement (54) according to any preceding claim characterised in that each filter element has a passband attenuation of less than 1 dB.

16. An arrangement for selecting a wavelength channel from radiation (66) which includes a plurality of wavelength channels, said radiation (66) being carried by an optical fibre (52), characterised by comprising: a radiation shaping arrangement (64), a tuneable optical filter arrangement (54) according to any of Claims 1 to 15 in which the plurality of filter elements (60a-60x) is selectively moveable into the path of said radiation, and a receiver (81), wherein the radiation shaping arrangement (64) receives the radiation (66) carried by the fibre (52) and shapes this into a beam (68) comprising at least a portion having a wavefront which is substantially planar, the beam (68) is directed onto a selected filter element of the filter arrangement (54) such that the substantially planar wavefront portion is incident on the selected filter element, and the filter arrangement (54) provides an optical output signal which is directed to the receiver (81).

17. An arrangement according to Claim 16 characterised in that the radiation shaping arrangement comprises one or more lens arrangements (64).

18. An arrangement according to Claim 17 characterised in that the lens arrangement or arrangements (64) act to collimate the radiation (66) into a beam (68) of radiation having a substantially planar wavefront and to direct the beam (68) onto the selected filter element.

19. An arrangement according to Claim 17 characterised in that the radiation (66) emitted by the fibre (52) has a substantially gaussian-shaped intensity profile, and the lens arrangement or arrangements (64) act to shape the radiation (66) into a beam (68) having a beam-waist and to direct the beam (68) onto the selected filter element, such that the beam-waist is incident on the filter element.

20. An arrangement according to any of Claims 16 to 19 characterised in that the receiver comprises an optical fibre (81).

21. An arrangement according to Claim 20 characterised in that the optical fibre (81 ) receives the optical output signal of the filter arrangement (54) and directs this to an optical to electrical converter (56).

22. An arrangement according to any of Claims 16 to 19 characterised in that the receiver comprises an optical to electrical converter.

23. An arrangement according to Claim 21 or Claim 22 characterised in that the optical to electrical converter comprises a photodetector (56) which receives the optical output signal of the tuneable optical filter arrangement (54), and converts this to an electrical output signal.

24. An arrangement according to Claim 23 characterised in that the optical to electrical converter comprises a preamplifying electronic circuit which receives the electrical output signal of the photodetector (56), which preamplifying electronic circuit is capable of receiving and processing signals comprising 2.5 Gbit per sec data, or 10 Gbit per sec data or 40 Gbit per sec data or any combination of such signals.

25. An arrangement according to any of Claims 16 to 24 characterised by further comprising apparatus for positioning and holding the filter elements of the tuneable optical filter arrangement (54) which apparatus positions and holds the selected filter element such that the angle between the substantially planar wavefront portion of the beam of radiation and the filter element is substantially 90°, and radiation comprising the wavelength channels which are not selected by the filter element are reflected from the filter element at an angle of substantially

180° and are directed back into the optical fibre (52, 100).

26. An arrangement according to any of Claims 16 to 24 characterised by further comprising apparatus for positioning and holding the filter elements of the tuneable optical filter arrangement (54) which apparatus positions and holds the selected filter element such that the angle between the substantially planar wavefront portion of the beam (68) of radiation and the filter element is other

than 90°, and radiation comprising the wavelength channels which are not

selected by the filter element are reflected from the filter element at an angle other than 180° and are directed into the end of another optical fibre (87).

27. An arrangement according to any of Claims 16 to 26 characterised by further comprising a second lens arrangement (70) which directs the optical output signal of the filter arrangement (54) to the receiver (81).

28. An arrangement according to any of Claims 16 to 27 characterised by further comprising a beam bending system (90) which acts to bend the radiation carried by the fibre (52) and emitted therefrom, or the optical output signal of the tuneable optical filter arrangement (54), or the radiation comprising the wavelength channels which are not selected by the filter element of the filter arrangement (54), or some or all of these.

29. An arrangement according to any of Claims 16 to 28 characterised by further comprising one or more shutters (82).

30. An arrangement according to Claim 29 in which a shutter (82) is placed between the radiation shaping arrangement (64) and the tuneable optical filter arrangement (54) which shutter (82), when operated, prevents radiation from the fibre (52) being directed onto the filter arrangement (54).

31. A wavelength division multiplex communication network characterised by incorporating at least one tuneable optical filter arrangement (54) according to any of Claims 1 to 15.

32. A wavelength division multiplex communication network characterised by incorporating at least one arrangement for selecting a wavelength channel according to any of Claims 16 to 30.

33. A wavelength division multiplex communication network according to Claim 31 or Claim 32 characterised by further comprising at least one coupling device (95, 97) for coupling the wavelength channels which are not selected by the or each tuneable optical filter arrangement (54) into said communication network.

34. A wavelength division multiplex communication network according to Claim 33 characterised in that the or each or some of the coupling devices comprise an optical circulator (95, 97).

35. A wavelength division multiplex communication network according to Claim 33 as dependent from Claim 32 characterised in that the or each or some of the coupling devices comprise setting up the arrangement for selecting length channel such that the filter elements of the tuneable optical filter arrangement are tilted with respect to the radiation from which a wavelength channel is to be selected.

36. A wavelength division multiplex communication network according to Claim 35 characterised in that the angle of incidence of the radiation on the filter elements is in the range 0.25° to 6°.

37. In a wavelength division multiplex optical communication system a method of selecting a wavelength channel from radiation including a plurality of wavelength channels comprising: providing a wavelength tuneable filter whose optical passband can be selectively tuned to a selected wavelength channel and directing said radiation through said filter; characterised in that the filter comprises a respective filter element for each wavelength channel wherein each filter element comprises a multiple-layered dielectric stack having a fixed wavelength passband corresponding to a wavelength channel and wherein a wavelength channel is selected by moving the respective filter element into the path of the radiation.